1. Field of the Invention
The present invention relates to a center-tap termination circuit in a differential signal transmission line for transmitting a high-speed differential signal.
2. Description of the Related Art
Low voltage differential signaling (LVDS) technology is used to suppress noise emissions when transmitting high-speed signals on a printed circuit board or using cables. FIG. 9 is a circuit diagram of a typical LVDS transmission model.
FIG. 9 depicts a ground 1100 (GND), a transmitting driver Integrated Circuit (IC) 1200, and a receiver IC 1300. Between the transmitting driver IC 1200 and the receiver IC 1300, a forward transmission line 1101 and a return transmission line 1102 are provided. The forward transmission line 1101 and the return transmission line 1102 constitute a differential signal transmission line for transmitting a low-voltage differential signal. In the differential signal transmission line, the forward transmission line 1101 and the return transmission line 1102 are designed to have the same electrical characteristics. In the vicinity of the input port of the receiver IC 1300, a resistor 1103 is connected between the forward transmission line 1101 and the return transmission line 1102. The resistor 1103 terminates the differential signal transmission line. In a case where the differential signal transmission line is designed to have a differential impedance of 100 Ω, differential mode current components flowing through the differential signal transmission line are completely matched and hence terminated by the resistor 1103 having a resistance of 100 Ω.
In addition to the differential mode current components, common mode current components 1110 flow through the differential signal transmission line shown in FIG. 9, which may cause noise emissions that cannot be ignored. Specifically, since the termination circuit shown in FIG. 9 has no line through which common mode current components 1110 can flow, the common mode current components 1110 return to the ground 1100 via a stray capacitance of the printed circuit board. Because this stray capacitance is unstable, cancellation of the common mode current components 1110 by return current becomes unstable, resulting in the emission of a large amount of noise. This emission of noise can cause serious problems, if the ground plane is defective.
To suppress the generation of such noise emissions due to common mode current components, Japanese Patent Laid-Open No. 11-205118 discloses a center-tap termination circuit. FIGS. 10A and 10B show this center-tap termination circuit used in an LVDS transmission model described in this document.
FIG. 10A is a circuit diagram of the center-tap termination circuit. Referring to FIG. 10A, in the vicinity of the input port of the receiver IC 1300, resistors 1104 and 1105 are serially connected between the forward transmission line 1101 and the return transmission line 1102. Between a connector that interconnects the resistors 1104 and 1105 and the GND 1100, a capacitor 1106 is connected. The resistors 1104 and 1105 and the capacitor 1106 constitute the center-tap termination circuit.
FIG. 10B is a plan view of the center-tap termination circuit shown in FIG. 10A, which is implemented on a printed circuit board. In FIG. 10B, components corresponding to those in FIG. 10A are given the same reference numerals.
Referring to FIG. 10B, reference numeral 1500 denotes a printed circuit board. The forward transmission line 1101 and the return transmission line 1102 are made from copper foil patterns. The resistors 1104 and 1105 have the same resistance, and are chip resistors, and capacitor 1106 is a chip capacitor. The forward transmission line 1101 and the return transmission line 1102 extend to the left of FIG. 10B and are connected to the transmitting driver IC (not shown). Similarly, the forward transmission line 1101 and the return transmission line 1102 extend to the right of FIG. 10B and are connected to the receiver IC (not shown). Since the resistors 1104 and 1105 and the capacitor 1106 are included in the termination resistor circuit, they are disposed in the vicinity of the receiver IC. The GND 1100 is provided around the forward transmission line 1101 and the return transmission line 1102.
First terminals of the resistors 1104 and 1105 are mounted on lands 1121 and 1123, respectively, and electrically connected to the forward transmission line 1101 and the return transmission line 1102, respectively. Second terminals of the resistors 1104 and 1105 are mounted on lands 1122 and 1124, respectively, inside the forward transmission line 1101 and the return transmission line 1102. The lands 1122 and 1124 are electrically connected to each other by wiring. Since the lands 1122 and 1124 are connected with each other, in conjunction with the forward transmission line 1101 and the return transmission line 1102, through a resistance of 50 Ω, the lands 1122 and 1124 correspond to the central potential of the differential signal transmission line. The lands 1122 and 1124 are electrically connected to a land 1125 by other wiring. A first terminal of the capacitor 1106 is mounted on the land 1125. Passing over the return transmission line 1102, a second terminal of the capacitor 1106 is connected to a land 1126 that is electrically connected to the GND 1100.
With the center-tap termination circuit described above, the forward transmission line 1101 is connected to the GND 1100 via the resistor 1104 and the capacitor 1106, and the return transmission line 1102 is connected to the GND 1100 via the resistor 1105 and the capacitor 1106. The common mode current components 1110 are thus negated by the impedance between the resistor 1104 and the capacitor 1106 and the impedance between the resistor 1105 and the capacitor 1106. With respect to the common mode current components 1110 on the differential signal transmission line, a path is ensured for return current 1120 flowing through the GND 1100. As a result, noise emissions due to the common mode current components 1110 are suppressed.
Since the forward transmission line 1101 is connected to the return transmission line 1102 through the resistors 1104 and 1105, the differential mode current components are completely matched with each other and are thus terminated, as shown in FIG. 9. To this end, the resistances of the resistors 1104 and 1105 should be chosen so that the combined resistances of the serially-connected resistors 1104 and 1105 are approximately equal to the differential impedance of the differential signal transmission line consisting of the forward transmission line 1101 and the return transmission line 1102.
Due to recent increases in the speed and frequency of signals used in differential signal transmission lines, noise emissions due to common mode current components generated by the differential impedance mismatching of the differential signal transmission line have become a problem. In particular, differential impedance mismatching due to variations in distance between a forward transmission line and a return transmission line, which constitute the differential signal transmission line, has become a serious problem.
A center tap termination circuit is included in the differential signal transmission line shown in FIGS. 10A and 10B. As such, the distance between the forward transmission line 1101 and the return transmission line 1102 at the position of the center-tap termination circuit is much greater than the distance at any other position on the differential signal transmission line.
FIG. 11A is a sectional view of the printed circuit board taken along line 11A—11A of FIG. 10B, and FIG. 11B is a sectional view taken along line 11B—11B of FIG. 10B. Generally, the differential impedance of the differential signal transmission line changes in accordance with the distance between the forward transmission line 1101 and the return transmission line 1102, the distance between the forward transmission line 1101 and the GND pattern, and the distance between the return transmission line 1102 and the GND pattern in the sectional view of the printed circuit board.
FIGS. 11A and 11B depict the capacitive coupling 1151 between the forward transmission line 1101 and the return transmission line 1102, the capacitive coupling 1153 between the forward transmission line 1101 and the GND 1100, and the capacitive coupling 1152 between the return transmission line 1102 and the GND 1100. Referring to FIG. 11B, the capacitive coupling 1152 and the capacitive coupling 1153 are not greatly changed from those shown in FIG. 11A, whereas the capacitive coupling 1151 is greatly reduced from that shown in FIG. 11A. Since smaller capacitive couplings generate larger differential impedances, the differential impedance at line 11A—11A is not the same as the differential impedance at line 11B—11B. As such, common mode current components are generated due to signal reflections.
FIG. 12 is a graph showing the waveforms of differential signals in the above case in which time is shown on the abscissa, and voltage is shown on the ordinate. Referring to FIG. 12, reference numeral 600 denotes a signal transmitted through the forward transmission line 1101, and reference numeral 700 denotes a signal transmitted through the return transmission line 1102. As is clear from FIG. 12, the transmission return/transmission forward (tr/tf) characteristics of the signals 600 and 700 do not match each other due to differential impedance mismatching, causing skew. As a result, many common mode current components are generated on the differential signal transmission line, resulting in increased noise emissions.
In a case where a plurality of chip components are mounted on a printed circuit board, the chip components need to be separated from one another by a predetermined distance due to the mechanical limitations of a mounting machine. In the center-tap termination circuit shown in FIGS. 10A and 10B, there is a “dead space” between the resistors 1104 and 1105. The larger the distance between the forward transmission line 1101 and return transmission line 1102, the greater the differential impedance, resulting in a further increase in common mode current components. The area in which the center-tap termination circuit is mounted is increased, resulting in a smaller degree of freedom in circuit design.